In this project, we have three main approaches. Approach 1: The p12 derived cleavage product p8 traffics to the cell surface, decreases TCR signaling and increases virus transmission. A balanced production of p8 and p12 is found in transfected cells. We hypothesized that perturbation of the relative amount of p8 and p12 may affect HTLV-1 infectivity or persistence. In HTLV-1 infected individuals, we found polymorphisms in orf-I associated with a decreased cleavage, Glycine at position 29 to Serine (G29S) that results in more p12 than p8, increased cleavage, Aspartic acid at position 26 to Asparagine (D26N) that results in more p8 than p12 and balanced production, Aspartic acid at position 26 (D26), that results in similar expression of both p8 and p12. Most HTLV-1 infected people (33%) and the biologically active HTLV-1 molecular clones available have Aspartic acid at position 26. We found a significantly higher viral burden in the blood of patients with polymorphisms compatible with a balanced production of p8 and p12 than in those with more p8 or more p12, suggesting that the production of both proteins is optimal for the establishment and/or maintenance of high viral burden. Mutations G29S and D26N introduced within the orf-I of an HTLV-1 molecular clone confirmed that both p12 and p8 are necessary to establish and maintain HTLV-1 infection in macaques. Approach 2: Ablation of orf-I and orf-II expression impairs HTLV-1 replication in dendritic cells and monocytes in vitro and in macaques in vivo. p30 inhibits the transcription of Interferon-responsive genes following triggering of TLR3/4 on the cell surface but not of TLR7/8 signaling in the endosome, whereas, p8/p12 affects both TLR3/4 and TLR7/8 signaling. These data suggest that the inhibition of innate responses by these viral proteins is likely essential for virus entry and egress and that these proteins may affect monocyte/dendritic cell function. We hypothesized that HTLV-1 infection of monocytes in vivo alters monocyte function that, in turn, may favor maintenance of high virus burden. The analysis of sorted monocytes from the blood of HTLV-1 infected individuals demonstrated various levels of viral DNA within the monocyte subsets. We found that there was a positive correlation with intermediate CD14+CD16+, as well as, non-classical CD14-CD16+ monocytes and virus burden, particularly in TSP/HAM. These data suggest that HTLV-1 infection of monocytes may promote their differentiation to pro-inflammatory monocytes. We will investigate the effects of p8/p12 and p30 using the whole virus or as individual proteins on chemo-taxis, phagocytosis and cytokine production using THP-1 as a model cell line in vitro. We plan to extend these functional studies by examining in vitro monocytes infected with HTLV-1 mutants and ex vivo infected monocytes obtained from HTLV-1 infected individuals with various levels of viral burden. Approach 3: We hypothesized that STLV-1 infection of macaques could be exploited as a model of HTLV-1 infection. We found that in STLV-1 infected macaques there is a significant increase in intermediate and non-classical monocyte populations. Future work will assess whether these monocyte subsets are also infected by STLV-1 and whether this animal model is suitable for testing strategies to decrease viral burden with drugs or immune modulators.